What is Required to Do Good Dairy Nutrient Management?

Larry Schwankl, Irrigation Specialist, Carol Frate, Farm Advisor, and
Deanne Meyer, Waste Management Specialist, UCCE

It is easy to get lost in the details of managing the nutrients on a dairy. That’s why it’s so important to understand the big picture of what we are trying to achieve. The overall nutrient management objective established in the General Order for Existing Milk Cow Dairies (General Order), by the Central Valley Regional Water Quality Control Board, is to apply no more than 140% of the nitrogen removed in the crop.

Two major forms of nitrogen are in manure water as it leaves the pond(s): ammonium and organic. Neither of these nitrogen forms will leach during the irrigation event in which they are applied. After the irrigation event, the ammonium nitrogen in the soil will be converted to nitrate in a matter of days (during warm weather). Change in the organic nitrogen form is slower as it must be mineralized to ammonium and then converted to nitrate. Nitrate moves readily with water through soil, so all nitrate present in the root zone at each irrigation is susceptible to being moved below the root zone (leached) when over-irrigation occurs and water drains below the root zone. Nitrate leached below the root zone cannot be used by the plant as a fertilizer, and it has the potential to contaminate groundwater.

There are two irrigation scenarios that may occur. First, when good irrigation water management occurs, the amount of water applied is close to that required to refill the crop’s root zone and minimal drainage or deep percolation is generated. Drainage or deep percolation will not occur as long as the applied irrigation water is held in the crop’s root zone. With good irrigation practices, it is important to apply the correct amount of nitrogen to supply the crop’s needs and to meet the General Order’s 140% nitrogen application criteria.

Second, if irrigation practices do result in drainage past the crop’s root zone, nitrogen must be applied carefully so that nitrate is not present in the root zone when drainage occurs. This requires that only enough nitrogen be applied at each irrigation to meet the crop’s needs until the next irrigation event. It’s important that the nitrogen also be in a form plants can use. Nitrogen available to the plant is the ammonium nitrogen from the recent irrigation event (either “as is” or as the quickly converted nitrate form), and a portion of the organic nitrogen that has been converted to ammonium or nitrate nitrogen. Precise and careful nitrogen applications are needed to match application form and timing with crop nutrient needs.

Both of these irrigation scenarios require that you:

1. Know the nutrient content of the manure water. Manure water samples will need to be tested and data incorporated into determining application rates.
2. Know the flow rate of manure water application. A reliable, easy to use flow measurement method, such as a flow meter, will need to be in place.
3. Know the nutrient content of irrigation water. A sample of the irrigation water will need to be tested.
4. Know how much irrigation water is applied. Reliable flow measurement methods need to be in place. For example, propeller flow meters on freshwater pump discharges provide good information, or your irrigation district may be able to provide good flow measurement information for water they supply.
5. Know how much water the crop needs and when it needs it. Use a reliable irrigation scheduling method, such as Evapotranspiration (ET) scheduling to determine how much irrigation water is required.
6. Know how much nitrogen the crop needs and when it needs it.

It is very important to have your flow measurement infrastructure (e.g. flow meters) in place and the procedure (who is going to read them and when they will be read) for using them worked out before the hectic irrigation season begins. You need to make sure the flow meter readings get recorded in a safe place where they can be found later. The data will be useful as you schedule nutrient applications. It will also be required to complete your annual report to the Central Valley Regional Water Quality Control Board.

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